THE URINARY ORGANS 65 



that the organ is composed of numerous lobules, 

 and that each lobule consists of a peripheral 

 or cortical substance and a central or medullary 

 substance (Fig. 32). Both substances contain 

 small, branched uriniferous tubules, by the 

 activity of which the constituents of the urine 

 are derived from the blood. In the cortical 

 substance the tubules are richly convoluted 

 and lined by a layer of granular, polyhedral 

 epithelial cells (Fig. 33). In this substance, 

 also, are tufts of capillary blood-vessels — the 

 glomeruli — each contained within a thin-walled 

 capsule which is really the commencement 

 of a uriniferous tubule. The tubules of the 

 medullary substance are narrower and straight 

 and lined by a clear, cubical epithelium sur- 

 rounding a relatively wide lumen (Fig. 34). 

 The straight tubules act as ducts which carry 

 the urine into the radicles of the ureter. 



The whole kidney is highly vascular in con- 

 formity with its function as a purifier of the 

 blood. Capillary vessels form the glomeruli, 

 and in addition are disposed as a network 

 between the uriniferous tubules. 



